Reforming of a naphtha fraction is generally accomplished by passing the naphtha through a reaction zone having one or more reactors. These reactors may contain one or more fixed beds of catalyst comprising a hydrogenation-dehydrogenation component supported on a porous solid carrier. Typical catalysts include platinum on alumina with or without such promoters such as rhenium, tin, iridium, etc. When a plurality of reactors is used, the naphtha fraction to be reformed is contacted in the first reactor with a platinum-containing catalyst at reaction conditions to convert principally naphthenes to aromatics. In addition to naphthene dehydrogenation, side reactions such as isomerization, hydroisomerization and hydrocracking may also occur. Typically, the effluent from the first reactor is heated prior to being introduced to a subsequent reactor.
After a period of use in reforming, the catalyst becomes gradually deactivated due to the deposition of coke on the surface of the catalyst and consequently a decrease of the octane values of the reformate product is observed.
If the octane requirements imposed upon the particular reforming system are to be continuously met, the reaction temperature of the catalyst must be increased in order to compensate for the loss in activity due to the coke deposition. The fastest catalyst deactivation occurs in the reactor where paraffin dehydrocyclization and hydrocracking are the principal reactions. Consequently, even with a constant inlet temperature, the average reaction temperature increases with each successive reactor, because the reactions in each successive reactor are not as endothermic as in the preceding reactor.
Coke deposition on the catalyst not only decreases the activity of the catalyst but also results in a decrease in the yield of C.sub.5 + gasoline product produced. Thus, the yield of C.sub.5 + gasoline product generally declines throughout the reforming process until it reaches an unacceptable level, at which point common practice is to regenerate all or part of the catalyst. Typical coke levels on the catalyst at the time of regeneration are 10 to 12 weight percent or more on the catalyst in the last reactor and 5 or 6 weight percent on the catalyst in the first reactor. Coke levels on catalysts in intermediate reactors will generally fall between these two figures.